[TenTec] RF susceptibility

[Stuart Rohre]

Any susceptibility to RF that gets back into the rig, starts with the 
quality of shielding (or not) of the headphone cords.  Many modern 
headphones do not even use shielded cords, nor twisted cords like the 
communications phones of yesteryear.  They have thin plastic housings 
offering no shielding to the transducer coils.

In modern Audio output circuits, (of the transformerless type), you can not 
put a bypass cap for RF on the speaker or headphone cord, lest it cause a 
high frequency condition in the Wideband IC audio amp chip.  In the old 
days, almost every lead leaving a chassis would simply have had  a disc 
ceramic cap for RF suppression.  Ferrites cost more than ceramic caps, thus 
are reserved to be added by the user "if needed".

Using balanced antennas, some distance from a properly laid out shack, 
usually results in no RF in the shack problems.  Some folks are more 
fortunate in this than others.  A shotgun approach to RF suppression ups the 
price.  RFI work usually has to be reactive to the problem, if any.  (As 
much as those of us who worked in RFI/ EMC would wish designers to think of 
all RF issues at the design stage).

Stuart
K5KVH